This invention relates to a process for producing plastic layers on gap sealing surfaces. The plastic surfaces are unaffected by temperature changes.
The efficiency of turbo engines is a direct function of the gap size between the housing and the rotor. A minimal gap size is achieved, for example, by a touching action of the blade tips on the gap sealing surfaces. When the blade tips touch the gap sealing surfaces, damage to the blade must be avoided. Plastic layers on gap sealing surfaces have the purpose of rendering the gap widths in an engine abradable and therefore adjustable without any significant abrasion of the blade tips. For this purpose, the plastic material is converted to a viscous mass and is knifed onto the stressed housing rings. In a further state of the art, prefabricated plastic inlet coatings are glued onto the housing rings. The adhesive and the primer must then harden. The plastic inlet coatings may also be cast on.
One disadvantage of these processes are high manufacturing expenditures, in the case of which a base must be applied after the cleaning onto which, after a heat treatment step, the plastic mass is glued, knifed or cast, and is then hardened. Then a cutting process takes place. Defects in the coating which become visible must be knifed out and hardened as well as refinished.
In the case of knifed, glued or cast-on plastic masses, the present maximal usage temperature is 180.degree. C.
The preparation of the individual components for the knifing or gluing requires additional manufacturing expenditures, in which case it is a disadvantage that the components have a limited storage time. Knifed, glued or cast-on plastic masses cannot be varied with respect to the buildup of their coating and are usually dense throughout the mass. This may have a harmful effect on the abrasion behavior during the touching action.
It is an object of the present invention to provide a process of the above-mentioned type by means of which, in one processing step, a gradual transition from a dense to porous mass can be provided in the plastic coating close to the surface for gap sealing surfaces. Thus, the disadvantages of the previous processes are overcome.
This object is achieved in that a plasma or acetylene oxygen flame is generated by a plasma or flame spraying burner and, below a cooling gas flow of from 40 to 150 SLpM, the flame melts a plastic mass and sprays it on a gap sealing surface.
It is an advantage of this process that coatings can be generated which have different porosities and hardnesses, and the coating characteristics can be still varied during the manufacturing operation so that the degree of porosity can be changed from 0 to 85% by volume in one operation and in one layer. Furthermore, the plastic structure created after the spraying is extremely homogeneous. Finally, this process is faster and therefore also lower in cost than the gluing, knifing or cast-on processes.
Advantageously, the strength of the cooling gas flow during the spraying time is varied for manufacturing sprayed layers that vary from being denser to porous. This has the advantage that no additional devices are required and no bubble-forming or foaming agents must be added to the plastic material.
In a preferred implementation of the invention, the plastic mass is added in powder form. This powder form is particularly advantageous in the case of flame spraying because powder particles can be converted uniformly into particle droplets by means of the flame.
In a further preferred implementation of the invention, the plastic mass is added as plastic wire. Plastic wire is advantageously suitable for a flame spraying process because the plastic droplets do not tear off the tip of the wire and spray onto the surface to be coated before they are in a highly plastic or liquid condition.
As the preferred plastic mass, polyphenylene sulfides are used which advantageously permit usage temperatures of up to 260.degree.. This is because a softening of this plastic mass does not start before 275.degree. C. is reached.
Preferably, up to 60% by volume fillers may also be added to the plastic mass. These fillers improve the touching behavior of the mass.
Preferably, calcium fluoride, zince oxide, magnesium oxide, or mixtures thereof are added as fillers. These are used particularly advantageously if, in the start-up phase of the engine, the blade tips are to be ground-in on the gap sealing surfaces.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.